As an example of an optical information recording medium which reproduces information by using laser beams, there is an optical disk. An optical disk is characterized to have a large capacity, and it is used widely as a medium for circulating and keeping images, music, or information on computers. Optical disks can be classified into “read only memory (ROM) type” which only reproduces information recorded in advance, “write-once read-many type” to which information can be written only once, and “rewritable type” to which information can be rewritten as many times as desired.
The capacity of the optical disk is determined according to the size of recording marks. The smaller the mark is, the more the capacity can be increased. The size of the recording mark basically depends on a condensed light spot size of a laser beam used for reproducing information. That is, with a smaller spot size, information of still higher density can be reproduced with a finer reproduction quality.
The spot formed by condensing laser beams by an objective lens does not converge into one point even at the condensed point and has a limited size due to a diffraction effect of light. This is generally called a diffraction limit, and it is a limit of mark length reproduced by λ/(4NA) where the wavelength of the laser beam is λ and the numerical aperture of the objective lens is NA.
For example, with an optical system where λ=405 nm and NA=0.85, 119 nm is the reproduction limit of the mark length, and a mark with length shorter than that cannot be read out precisely. In order to increase the capacity of the optical disk, there is a method of shortening the wavelength of the laser beam or a method of increasing NA of the objective lens. However, there is a limit in shortening the wavelength of the laser beam and increasing NA of the objective lens in terms of manufacture of optical components.
In the meantime, as a technique for improving the reproduction resolution by going over the diffraction limit, there is known a medium super-resolution technique. In the medium super-resolution, used is a medium which utilizes a super-resolution film whose optical characteristic changes nonlinearly by temperatures or light intensities. Here, a case of using a super-resolution film whose reflectance drastically changes at a given temperature as depicted in Patent Document 1 will be described by referring to FIG. 11. In this case, a phase change material is used as the super-resolution film, and a difference between the reflectance in a crystal state (solid phase) and the reflectance in a state where it is fused at a melting point or higher (liquid state) is utilized.
FIG. 12 is a conceptual chart showing a fragmentary enlarged view of a single track recording mark sequence out of recording marks formed in advance along a spiral track on a transparent substrate of a super-resolution optical disk according to a widely-used technique. A laser beam passed through an objective lens is irradiated on a medium as a condensed light spot 200. The temperature in the vicinity of the condensed light spot 200 is increased due to absorption of the irradiated laser beam, so that a high-temperature region is generated. The light intensity distribution in the condensed light spot 200 is Gaussian distribution where the intensity in the center is strong. Thus, regarding the temperature distribution that depends on the light intensity, the temperature in the vicinity of the center becomes the highest.
In a fused region 201 that is a region particularly exceeding the melting point of the super-resolution film among the high-temperature region, the reflectance is increased since the super-resolution film changes from a solid phase state to a liquid phase state. Thus, it can function as an aperture for reproducing the recording mark. As a result, the size of the aperture contributing to reproduction can be made smaller than the condensed light spot size that is determined according to the diffraction limit Therefore, information of a small recording mark 203 that is equal to or smaller than the reproduction limit can be read out as a super-resolution reproduction signal.
As in this case, a super-resolution reproduction method with which apertures are formed in the rear side of the traveling direction of the condensed light spot is called RAD (Rear Aperture Detection) type super-resolution. Since the optical disk is rotating at a high speed, the fused region 201 moves on the optical disk. Thus, the time for a given region to be fused is extremely short.
With the RAD type super-resolution of a ROM disk, an ideal super-resolution reproduction can be done only by using apertures when reflectance Ru of the aperture (fused region 201) is sufficiently higher than reflectance Rd of regions (non-fused region 202) other than apertures. However, practically, the reflectance in the regions other than the apertures cannot be made so small that it can be ignored. Thus, in Patent Document 1, the bit error rate is set to be equal to or less than “10^−5” by setting the ratio of the two kinds of reflectance as Ru/Rd≧8. In this Specification, “10 to the power of minus five” is expressed in a form of “10^−5”.
Further, as other technical documents related to this, Patent Document 2 discloses an optical recording medium which utilizes a non-linear optical thin film whose refractive index changes reversibly according to changes in the intensity of irradiated laser beams. Patent Document 3 discloses an optical recording medium in which a light adjusting film is provided between a transparent substrate and a recording film so as not to generate a phase difference between a recorded section and unrecorded section. Patent Document 4 discloses an optical recording medium in which deterioration in the performance for repeated reproduction is suppressed by setting the melting point of a super-resolution layer to be lower than that of a recording layer.
Patent Document 1: Japanese Unexamined Patent Publication Hei 09-128807
Patent Document 2: Japanese Unexamined Patent Publication 2003-195374
Patent Document 3: Japanese Unexamined Patent Publication Hei 08-147757
Patent Document 4: Japanese Patent Application Publication 2008-511096